1. Field Of The Invention
This invention relates to the field of two-way communication devices in which one such device is passive and, more particularly, to two-way communication devices in which one device is a stationary device and the other device is a portable passive device.
2. Description Of Related Art
It is well known to use two-way communication devices utilizing a stationary member and a portable passive member. Such portable passive members are generally radio frequency (RF) tags. An RF tag uses radio frequency energy to remotely identify itself to a reader, which may be several inches to many feet away. The tag may be mounted on a container for cargo management or carried by a person for security access. It is preferable that the tag not only be extremely inexpensive but also rugged and reliable.
FIG. 1 shows a typical RF tag system known in the prior art. Therein, a reader 10 and tag 12 engage in two-way communication. Power 14 is imported into reader 10 and transmitted as RF energy signal 16 to tag 12. After processing by tag 12, tag 12 transmits RF modulated signal 18 which is received by reader 10. Utilizing this RF modulated signal 18, reader 10 obtains tag ID code 20 which allows it to identify tag 12.
For security access applications, low frequency tags having a frequency less than one MHz are preferred. Such tags provide the advantage of permitting through-the-body operation and easy clock generation. For short read ranges on the order of three feet or less, such a tag operates without a battery, using only the energy absorbed from the reader's RF field. The energy is coupled from the reader to the tag magnetostatically. This coupling is a near-field phenomenon which is analogous to an air core transformer having a very low coupling coefficient.
Since there is no external power supply in the circuit from the reader antenna to the tag and back to the reader, the signal level returning from the tag to the reader is much lower than the transmitted signal from the reader. It has been found that in the three foot read range, the returning signal from tag to reader is more than 80 dB less than the transmitted signal from the reader to the tag. This large signal differential requires a large dynamic range in the reader receiver or a frequency conversion in the tag so that the received frequency at the tag is different from the transmitted frequency. Frequency conversion enables front-end filtering in the receiver of the transmitted frequency. At the low frequencies desired for RF tag operation, frequency conversion is a simple function performed by digital circuitry. Accordingly, a dual frequency RF tag system is desirable.
The power received at the tag may be extremely low. It is not infrequent for such power to be less than 100 .mu.W. However, this is enough power to operate a digital CMOS integrated circuit as long as a relatively large voltage in the range of greater than 3 volts is generated. In order to conserve power in the tag, the voltage needed to transmit the signal from the tag should be minimized. Consequently, there is a need for an antenna circuit capable of deriving large voltages from the reader signal but conserving power in the transmission of the return signal.